High performance composite products such as those used in the aerospace and aviation industry can take the form carbon-carbon composite materials. Carbon-carbon composite products are derived from high carbon content fibers which are arranged in laminate plies of fabrics or tapes and integrated into a composite structure by means of a carbonaceous matrix. The tapes, consisting of a plurality of unidirectional carbon strands and the fabrics, composed of woven warp and fill strands, are formed from strands, sometimes referred to as yarns or tows, of bundles of discrete carbon fibers of a very small diameter, typically 4-12 microns. A single tow may be formed of hundreds or thousands of individual fiber filaments. The carbon fibers may be resin based fibers such as polyacrylonitrile (PAN) or rayon or they may be pitch-based fibers. The pitch based fibers are generally produced from mesophase pitch and normally have an extremely high carbon content, for example about 99+%. The resin-based carbon fibers also have a high carbon content, although somewhat less than that of the pitch based fibers. PAN fibers have a carbon content of about 92-99% whereas rayon fibers have a carbon content of about 97-99%. The matrix material is generally derived from thermoset resins such as furan polymers or phenol formaldehyde polymers, although pitch based materials may also be employed to supply the composite matrix.
Carbon fibers and fabrics useful in the formulation of carbon-carbon composites are described Kirk-Othmer, "Encyclopedia of Chemical Technology," 3rd Ed., 1978, Volume 4, pages 622-631 under the heading Carbon (Carbon and Artificial Graphite). Characteristics are described in Kirk-Othmer for rayon-based, PAN-based and pitch-based and mesophase pitch-based fibers. As described there, properties such as Young's modulus (the modulus of elasticity), electrical and thermal conductivity, and tensile strength depend on the degree of preferred orientation. Very highly oriented carbon fibers possess a Young's modulus close to that of a single graphite crystal. Electrical and thermal conductivity of the fiber also increase with increased orientation. The Kirk-Othmer article also describes the formation of graphitized carbon and glassy carbon.
High performance composites, including those based upon carbon fibers, and their preparation are also described in the Supplement Volume of Kirk-Othmer, 3rd Ed., in pages 260-281 under the heading "Composites, High Performance." A suitable fabrication sequence for producing composite products is described in pages 266-270. This Kirk-Othmer article describes the sequence of the combining of fibers with a matrix material and the subsequent stacking, bagging and curing of the laminate plies to produce a composite product. As described there, a plurality of laminate sheets of prepreg tape are stacked to form a laminate layup on a forming surface. A nylon bag is placed over the layup and sealed at its edges. A vacuum is then applied to the sealed assembly in order to evacuate air from the layup. The assembly is then placed in an autoclave and heated and pressurized to effect a cure cycle.
As described in the above-mentioned Kirk-Othmer, vol. 4, on page 626, 627, mesophase pitch-based fibers have a relatively high modulus of elasticity. The use of these fibers in forming carbon-carbon composites structures useful as structural materials, thermal radiators, or antennas is also disclosed in Rubin. L, "High-Modulus Carbon Fiber Based Carbon-Carbon for Space Applications, 1986, Report No. SD-TR-86-45 Space Division, Air Force Systems Command, Los Angeles Air Force Station, Los Angeles, Calif. 90009-2960. In Rubin, a carbon-carbon composite structure was produced from a flat layup of 16 plies of fabric woven from a pitch based carbon yarn obtained from Union Carbide Corporation and identified as P-100 yarn. .The laminate plies were in the form of a fabric woven from the yarn in an 8-harness satin weave having a linear density (end count) of 20 yarn ends per inch in both the warp and fill directions. In the Rubin procedure, the matrix material was formed from A240 petroleum pitch available from Ashland Oil Company. Processing included heating the preform layup to 540.degree. C. at 100 psi in a press under a nitrogen atmosphere in order to pyrolyze the matrix pitch. The composite was then heated to 1500.degree. C. in argon, reimpregnated with the pitch and the pyrolysis step repeated. The product was then graphitized by heating at a temperature of 2200.degree. C. under an argon atmosphere. At the conclusion, the product was treated with carbon vapor deposition at 1100.degree. C. in order to reduce porosity and increase strength. The Rubin article reports on thermal conductivity, diffusivity thermal expansion, and electrical resistivity of the product.
U.S. Pat. No. 4,659,624 to Yeager et al. discloses carbon-carbon composite products in which desired structural properties are provided along a predetermined direction of load through the use of carbon fiber unidirectional tapes either alone or in combination with fabrics. Yeager discloses that strength tailoring of the product can be achieved during the initial layup and that increased strength generally will be obtained in the direction parallel to the longitudinal direction of the tows of the unidirectional tape plies. In forming the product, after the plies are laid up with the desired resinous matrix material, an initial cure cycle is undertaken to bind the plies together. Thereafter a pyrolysis cycle is instituted in order to carbonize the matrix material. This may involve a cycle of about three days in which a final temperature of about 1500.degree. F. is reached. Thereafter, the product is subjected to a densification procedure which can be accomplished through liquid impregnation or through chemical vapor deposition. The preferred technique is liquid phase impregnation in which a carbonizable liquid such as furfuryl alcohol or a phenolic based resin is used to impregnate the cured and pyrolized carbon-carbon material. As described in the Yeager patent, densification can be carried out by introducing the liquid phenolic resin into the product under an applied pressure of about 80 to 100 psi for a period of about 30 minutes. The product is then heated to a pyrolysis temperature of about 1500.degree. F. The densification procedure can be repeated up to about 5 times to obtain the desired density and strength of the product.